Magnetic recording media are today desired to have not only enhanced recording capacity but also improved reliability and durability. An intense effort is therefore underway to boost the recording capacity of next-generation magnetic recording media by using short wavelength signals to realize high recording density. The achievement of this goal will require a magnetic powder consisting of fine particles and exhibiting excellent properties. Unless the particle size is much smaller than the length of the region for recording the short-wavelength side signal, a distinct magnetic transition state cannot be produced, making practical recording impossible. The particle size of the magnetic powder is therefore required to be sufficiently small.
To realize higher density, the resolution of the recording signal must be increased. Reduction of magnetic recording medium noise is therefore important. Noise is usually attributable to particle size. The finer the particles are, the lower the noise becomes. This also makes it necessary for a magnetic powder used for high density recording to have sufficiently small particle size.
Moreover, a magnetic powder used in a magnetic recording medium enabling high density recording requires high coercive force (Hc) in order to maintain magnetism in the high-density medium and ensure the output. In addition, switching field distribution (SFD) must be made as narrow as possible because low SFD is essential for realizing high-density recording.
As the particle size of a magnetic powder becomes finer, dispersion during coating material preparation at the time of tape making becomes more difficult for a number of reasons, including that the specific surface area increases, interparticle sintering occurs and interparticle aggregation arises. Good dispersion is therefore required even when the particles are fine. In light of these circumstances, a magnetic powder for a magnetic recording medium is required from the functional aspect to simultaneously exhibit ultra-fine particle size, high coercive force, low SFD and excellent dispersibility.
On the other hand, the importance of avoiding damage to stored data increases with increasing storage capacity of the magnetic recording medium. A tape or the like for data storage is therefore required to have enhanced reliability. A tape with improved storage stability is therefore desired. In other words, it is important for the particulate magnetic powder itself to have excellent weatherability and oxidation resistance enabling it to be stored stably unaffected by the ambient environment.
A magnetic powder suitable for a high-density magnetic recording medium is therefore required to simultaneously meet the requirements not only of ultra-fine particle size, high Hc, low SFD and excellent dispersibility but also of good weatherability and oxidation resistance. However, no magnetic powder simultaneously and fully meeting these requirements has yet emerged.
Progress in the particle refinement of a magnetic powder is accompanied by the problem of a very large rise in particle surface activity. In the case of a metallic magnetic powder, the practice is to oxidize the surface of a magnetic powder obtained by reduction by some method so as to impart oxide stabilization by putting it in a condition formed with an oxide coating. Various surface oxidizing methods are available, including the method of gradual oxidation of the surface under an appropriate temperature in the presence of an oxygen-containing gas and the method of emersion in toluene or the like followed by dry-oxidation in air, but in all such methods, surface oxidation with respect to ultra-fine particles degrades the acicularity of internal metallic portions, markedly lowers Hc and causes SFD deterioration.
With such a technique that performs surface oxidation, the oxidation resistance of the obtained magnetic powder improves with increasing oxidation, i.e., with increasing thickness of the surface oxide film, but on the other hand, the acicularity of the internal metallic core portion markedly deteriorates, so that the coercive force (Hc) of the magnetic powder, which depends on the magnetic shape anisotropy, markedly decreases and the SFD exhibits degradation. In addition, the volume of the metallic portion inside the particles declines in proportion as the surface oxidation of fine-particle metallic magnetic powder advances. With fine particles, a certain degree of decrease in volume causes a marked decline in magnetic properties owing to thermal fluctuation, and a further degrease causes lack of magnetism owing to superparamagnetism.
Also owing to these circumstances, an attempt to enhance oxidation resistance by means of strengthening the surface oxidation lowers Hc, degrades SFD and promotes superparamagnetism, so that the magnetic properties become unsuitable as those of metallic magnetic particles used in a high recording density magnetic recording medium. Further, this problem exists not only in metallic magnetic powders but also in iron carbide, iron nitride and the like in which a metastable phase is present, so that it is difficult solely by forming a surface oxide coating to provide sufficient weatherability and oxidation resistance while simultaneously realizing the needed magnetic properties.
Patent References 1 and 2 teach technologies for using a silane coupling agent to improve the weatherability of a metallic magnetic powder and improve the dispersibility of a metallic magnetic powder in resin.
Patent Reference 1JP-Hei-7-272254 APatent Reference 2JP-Hei-7-94310 A